In some radio-frequency information transmission applications, it has been found that the transmission or reception antenna could have an impedance that is highly dependent on external conditions, and notably dependent on the environment in which the antenna is placed.
By way of example, in medical telemetry, there may be cause to introduce the antenna into a probe placed in the human body, and the impedance is then highly dependent on the biological environment that contains the antenna. It depends on the electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscles, fat) or of the liquid environment (blood, other liquids) in which the antenna may be immersed.
Even in more conventional radio-frequency transmission applications (mobile telephony, etc.), the impedance of the antenna may vary.
In a general manner, antenna impedance variations are particularly sensitive for antennas of very small dimensions having a high quality coefficient, which are used in applications with high miniaturization constraints.
These impedance variations may cause losses called mismatch losses: these losses result from the fact that the transmission chain that feeds the antenna, or the reception chain that receives a signal from the antenna, is generally designed to have optimum performance levels when it is loaded (at the output for the transmission chain or at the input for the reception chain) with a quite specific nominal impedance; it has degraded performance levels when it is loaded with an impedance that is different from its nominal value. Mismatch losses can be as much as 40 dB.
For this reason, it is known practice to interpose, between the output of a power amplifier and the antenna of a transmission chain, an impedance-matching network that prompts the transmission chain to see a different impedance from that of the antenna and one that is preferably equal to the nominal value for which it has been designed, for example 100 ohms or 500 ohms. The matching network is tunable, i.e. the values of its capacitive and/or inductive elements are adjustable so as to take the environmental conditions of the antenna into account in order to perform the best possible matching whatever the circumstances. Likewise, it is known practice to interpose such an impedance-matching network between the antenna of a reception chain and the input of a low-noise amplifier.
Several techniques have been proposed for automatically tuning such a matching network, so as to keep up with, by way of example, variations in the antenna impedance that are caused by outside conditions.
The document U.S. Pat. No. 4,375,051 teaches the use, in a transmission chain, of a bidirectional coupler for detecting a mismatch by measuring the fraction of the power provided by an amplifier that is reflected by the antenna. This measurement serves to control the impedance network in order to modify its configuration in a way that tends to reduce the reflected power. This method suffers from two disadvantages: firstly, the reflected power may be low and subject to parasitic interference, because any interference picked up by the antenna comes to distort the measurement owing to the fact that it is added to the reflected power. Secondly, there is no one-to-one relationship between the quantity of reflected power, which serves as an input for the feedback control, and the complex impedance value with which the matching network would need to be provided in order to really match the amplifier to the antenna. This method therefore leads to a new impedance that is not necessarily optimum, because a plurality of pairs of complex impedances correspond to a given power.
Document US 2009/0130991 discloses a method for adjusting the values of the reactances of an impedance-matching circuit that is arranged between a reception antenna and a low-noise amplifier in which the reactances of the elements of said matching circuit are iteratively adjusted so as to maximize the intensity of the signal output from said low-noise amplifier. The convergence of the iterative optimization algorithm may be very slow.
The documents EP 2 037 576, WO2011/026858, EP 2 509 222 and EP 2 509 227 describe various variants of an automatic impedance-matching method for a radio-frequency transmission or reception chain, in which current and voltage measurements at the input (for a transmission chain) or at the output (for a reception chain) of the matching network allow the antenna impedance to be determined. The knowledge obtained in this manner about the antenna impedance allows conventional techniques, for example based on a Smith chart, to be used to adjust the impedance values of the elements of the matching network so as to achieve impedance matching.
These techniques operate well in transmission, but applying them to reception chains is much trickier. Specifically, the power of the signal output by the antenna is very low, and it is difficult to drain off a portion of it in order to take the measurements that make impedance matching possible; in the best case, this requires the use of high-precision detection circuits, which are very complex and expensive. To overcome this drawback, it has been proposed for the current and voltage measurements to be taken at the output of the antenna amplifier (low-noise amplifier, or LNA). However, this is not always possible since certain LNAs are designed in such a way as to have an output impedance that is substantially independent of their input impedance: in this case, measurements taken at the output of the amplifier do not allow the antenna impedance to be determined. Proceeding in a dichotomic manner has also been proposed, by regulating the matching circuit so as to maximize the power output by the LNA without measuring the antenna impedance beforehand. This solution is not satisfactory since, in the absence of special precautions, maximizing the output power may result in a very high noise level, which is not acceptable.
The documents US 2008/129610 and JP 2013/070143 disclose automatic matching techniques that allow both the gain and the noise level to be optimized. These methods proceed by trial and error; they are therefore relatively slow, and the operation of the reception chain is heavily disrupted for the duration of the matching phase.